LATERAL WALLS OF SIEVE-TUBE ELEMENTS 97 



method of development of tliese stractures. P'urthermore, the 

 terminology used in describing the adult sieve-plate and its 

 homologues is, as Esau (1939) has shown, in a confused state. 

 It does seem evident, however, that in some respects, sieve-plates 

 are fundamentally similar to simple pits (Esau, 1939, pp. 397- 

 399). In conifers, the sieve-plates arise directly from the large 

 primordial pits which are present on the radial walls of the young 

 sieve-cells. Also in many angiosperms, it is possible to trace the 

 origin of the sieve-plate to a primordial pit of a meristematic cell. 

 In some plants, however, such as Rohmia and Cucurhita, it does 

 not appear possible to refer the large solitary sieve-plates on the 

 end-walls to development from a single primary pit area. Esau 

 suggests that in Rohinia "one might assume that several shallow 

 primordial pits together form one sieve-plate, the single or 

 numerous plasmodesmata of one pit giving rise to one connecting 

 strand of a sieve-plate." During the development of the sieve- 

 plate, each plasmodesma or in gymnosperms each group of plas- 

 modesmata becomes surrounded by a cylinder of callus. The 

 chemical nature of this substance is still in question but callus- 

 cylinders are readily stained and differentiated by treating sec- 

 tions with aniline hluc. As the maturation of the sieve-tube ele- 

 ment progresses, the amount of callus on the sieve-plates increases 

 so that the originally separate cylinders become confluent or 

 fused and the plate becomes coated on both sides with callus. 

 According to Esau (1939, p. 391) this final accumulation is 

 " depiitive-c alius" and indicates "the approach of a functionless 

 state of the sieve-tube." There appears to exist no conclusive 

 evidence that definitive-callus blocks up the pores of the sieve- 

 plate. On the contrary, the plasmodesmata are noticeably 

 stretched and eventually die. In many plants, the definitive- 

 callus becomes dissolved away from the plate prior to the death 

 of the sieve-tube elements and their companion cells which even- 

 tually become obliterated or crushed by neighboring cells. 



3. Lateral walls of sieve-tuhe elements. The lateral walls of 

 recently differentiated sieve-tube elements are frequently thick 

 and glistening in untreated sections. They have been termed 

 "nacre" because of this appearance. Chemically, these walls 

 appear to consist of cellulose and probably are, morphologically. 



